1. Field of the Invention
The present invention relates to an image processing apparatus for processing an image signal.
2. Related Background Art
A procedure has been proposed whereby a pulse width-modulated binary signal is generated by comparison of an analog signal converted from a digital signal with a periodic pattern signal such as a triangle wave for obtaining a half tone gradation (or tonality) when a digital image signal is binary coded to produce an image using a laser beam printer or the like.
FIG. 2 is a specific example of the procedure.
A digital video signal is latched by an image clock video-CLK at a latch 1 to be synchronized. The signal is converted to a analog video signal by a D/A converter 2. The output of the D/A converter, after having been changed to a voltage level at a resistance 3, is applied to one input terminal of a comparator 4. Periodic signals such as a sawtooth wave and a triangle wave are generated from a pattern signal generator circuit 5 and are applied to another input terminal of the comparator 4, in which the signals are compared with the above-mentioned analog video signal and the output of the comparator 4 is a pulse width-modulated video signal. The video signal is applied to a modulation circuit for modulating laser light, which is turned on and off according to the pulse width to produce a half tone image on a recording medium (not shown).
The image clock and the pattern signal are synchronized with a known BD signal which indicates the scanning position of the laser beam. The pattern signal generated from the pattern signal generator circuit 5 is synchronized with the image signal. The repetition period of the pattern signal is usually of an integer multiple of that of the image signal. An optimal period is selected dependent upon the reproduced image, reproducing medium, or reproducing conditions such as a resolution. For instance, when an electronic photograph-typed laser beam printer having a resolution of 400 dots/inch intends to reproduce a sufficient gradation, such as 64 gradations, the period must be about three times that of the picture element. The period of the pattern signal which is made longer than that of the picture element for improving the reproducibility of the half tone is certainly effective for a half tone image but raises a problem of displaying, as shown in FIG. 3, notched-edged characters and broken lines in the image for characters fine lines.
FIGS. 4A and B are drawings for describing the above problem. Vxx' in the figures shows an image signal read out from the portion X-X' in FIG. 3. The signal is compared to a sawtooth wave shown in dotted lines superimposed thereon so as to pulse-width modulate the signal and obtain a reproduced image shown P.sub.1. In general, this P.sub.1 is preferably reproduced by two black lines. But, fine lines are broken so that a produced character is poor in quality.
FIGS. 4A and B are examples in which sawtooth waves of different periods are employed.
A countermeasure against such a problem raised has been includes an image signal indicating a character with a pattern signal of a short period, such as a sawtooth wave of a period equal to the picture element period thereby reducing conspicuousness of notched characters and broken lines. However such a procedure requires automatic, real-time determination whether the image represents a character or a half-tone. The realization necessitates a complex circuit and may raise a problem of a discrimination error.